It is known that nitrogen oxides can be removed from a flue gas stream before it is released into the atmosphere as a method of avoiding environmental pollution, by a selective noncatalytic reduction of the nitrogen oxides with ammonia.
Especially when a noncatalytic reduction is to be carried out, it is found that nonreacted ammonium can form a residue in the flue gas stream of the reaction chamber and might be discharged into the atmosphere. Another portion of the ammonia can react with acid compounds of the flue gas to form solid ammonium compounds, such as ammonium sulfate and ammonium chloride which are collected together with the flyash or otherwise may be released into the atmosphere.
In EP-A No. 2-0 264 041 a process is described for the recovery of ammonia in which the nonreacted ammonia and ammonium chloride are scrubbed from the flue gas and ammonia is driven with air out of the scrubbing liquid. The ammonia enriched air is either introduced into the combustion zone or upstream of the catalytic or noncatalytic reduction zone into the flue gas passage so that the recycled ammonia also participates in the nitrogen oxide reduction process.
When the recycled ammonia is fed into the combustion there is the danger that it will meet high temperatures at which it can be burned so that the nitrogen oxide levels will not be reduced, but rather are increased. When the recycled ammonia is blown into the flue gas, carrier air together with entrained water vapor may cause problems in the subsequent treatment of the flue gas.
German Patent Document No. 35 26 756 describes a process for the removal of ammonia residues from the flyash of a flue gas which can be then used for the selective reduction of nitrogen oxides. Here the flyash separated from the flue gas is treated with hot treating gas, e.g. air heated to 700.degree. to 800.degree. C., so that the ammonia is driven off into the treating gas. The ammonia is then recovered from the treating gas or the treating gas with the ammonia is introduced into the reaction zone charged with a catalyst for promoting the nitrogen oxide reduction.
A similar process in which the treating gas is flue gas at a temperature 350.degree. to 700.degree. C. is described in German Patent Document No. 30 32 927.
German Patent Document No. 24 11 672 indicates that in the selective noncatalytic reduction of nitrogen oxides in flue gas use may be made of ammonia which has been dissolved in water, i.e. ammonia water, and German patent Document No. 38 00 730 indicates that an aqueous ammonia solution can be used for both catalytic and noncatalytic processes. In the prior processes, where recovery of ammonia from the flue gas downstream of the reduction zone is effected, the quantity of recycled ammonia is dependent upon the operating conditions and it has been difficult to coordinate both the recycled and fresh ammonia additions to the flue gas. It can be safely said, therefore, that reliable control of the earlier processes could not be ensured by the techniques which were previously used.